Group 7: The Halogens and Halogen Displacement
Introduction to Group 7
The Group 7 elements are known as the halogens. They are: fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At).
All halogens have seven electrons in their outer shell and exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). They need to gain one electron to achieve a full outer shell, forming 1− ions called halide ions.
Physical Properties
The halogens show a clear trend in physical properties going down the group:
| Halogen | State at room temp | Colour | Melting point (°C) |
|---|---|---|---|
| Fluorine | Gas | Pale yellow | −220 |
| Chlorine | Gas | Green/yellow | −101 |
| Bromine | Liquid | Red/brown | −7 |
| Iodine | Solid | Dark grey/purple vapour | 114 |
Going down the group:
- Melting and boiling points increase because the molecules get larger, so the intermolecular forces (van der Waals) between molecules are stronger
- Colour gets darker
Reactions of Halogens
Halogens react with metals to form metal halides (ionic compounds):
2Na(s) + Cl₂(g) → 2NaCl(s)
2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)
2Fe(s) + 3Br₂(l) → 2FeBr₃(s)
Halogens react with hydrogen to form hydrogen halides:
H₂(g) + Cl₂(g) → 2HCl(g)
These dissolve in water to form acidic solutions (e.g. hydrochloric acid).
Reactivity of Halogens
Reactivity decreases going down Group 7. Fluorine is the most reactive, iodine is the least reactive of the common halogens.
Why Reactivity Decreases
As you go down Group 7:
1. The atoms get larger — more electron shells
2. The outer shell is further from the nucleus
3. There is more shielding from inner electrons
4. The nucleus has less attraction for an incoming electron
5. It is harder to gain the extra electron needed
6. So the element is less reactive
This is the opposite trend to Group 1, because halogens gain electrons while alkali metals lose them.
Halogen Displacement Reactions
A more reactive halogen can displace a less reactive halogen from a solution of its salt. This is because the more reactive halogen gains electrons more easily than the less reactive one.
Testing with Halide Solutions
| KCl solution | KBr solution | KI solution | |
|---|---|---|---|
| Chlorine water | No reaction | Orange/brown (Br₂ formed) | Brown (I₂ formed) |
| Bromine water | No reaction | No reaction | Brown (I₂ formed) |
| Iodine solution | No reaction | No reaction | No reaction |
Key Displacement Equations
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)
Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)
Ionic Equations
Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
In this reaction, chlorine gains electrons (it is reduced) and bromide ions lose electrons (they are oxidised). This is a redox reaction.
Hydrogen Halides and Halide Tests
To test for halide ions in solution, add a few drops of dilute nitric acid followed by silver nitrate solution:
| Halide ion | Silver halide precipitate | Colour |
|---|---|---|
| Chloride (Cl⁻) | Silver chloride | White |
| Bromide (Br⁻) | Silver bromide | Cream |
| Iodide (I⁻) | Silver iodide | Yellow |
Exam Tips
- Reactivity trends for Group 7 are the opposite of Group 1 — make sure you can explain both using atomic radius and shielding
- Displacement reactions are a common exam question — learn the pattern and be able to predict whether a reaction will occur
- Remember: a halogen higher in the group displaces one lower; never the other way around
- Learn the halide test colours: white, cream, yellow (in order Cl, Br, I)